Add global_asm! for module-level inline assembly

Author: Amanieu

text/0000-global-asm.md

@@ -0,0 +1,61 @@
+- Feature Name: global_asm
+- Start Date: 2016-03-18
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC exposes LLVM's support for [module-level inline assembly](http://llvm.org/docs/LangRef.html#module-level-inline-assembly) by adding a `global_asm!` macro. The syntax is very simple: it just takes a string literal containing the assembly code.
+
+Example:
+```rust
+global_asm!(r#"
+.globl my_asm_func
+my_asm_func:
+    ret
+"#);
+
+extern {
+    fn my_asm_func();
+}
+```
+
+# Motivation
+[motivation]: #motivation
+
+There are two main use cases for this feature. The first is that it allows functions to be written completely in assembly, which mostly eliminates the need for a `naked` attribute. This is mainly useful for function that use a custom calling convention, such as interrupt handlers.
+
+Another important use case is that it allows external assembly files to be used in a Rust module without needing hacks in the build system:
+
+```rust
+global_asm!(include_str!("my_asm_file.s"));
+```
+
+Assembly files can also be preprocessed or generated by `build.rs` (for example using the C preprocessor), which will produce output files in the Cargo output directory:
+
+```rust
+global_asm!(include_str!(concat!(env!("OUT_DIR"), "/preprocessed_asm.s")));
+```
+
+# Detailed design
+[design]: #detailed-design
+
+See description above, not much to add. The macro will map directly to LLVM's `module asm`.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Like `asm!`, this feature depends on LLVM's integrated assembler.
+
+# Alternatives
+[alternatives]: #alternatives
+
+The current way of including external assembly is to compile the assembly files using gcc in `build.rs` and link them into the Rust program as a static library.
+
+An alternative for functions written entirely in assembly is to add a [`#[naked]` function attribute](https://github.com/rust-lang/rfcs/pull/1201).
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None